install.texi (Configuration): Document the --with-long-double-format={ibm,ieee}…

install.texi (Configuration): Document the --with-long-double-format={ibm,ieee} PowerPC configuration options.

2018-02-07  Michael Meissner  <meissner@linux.vnet.ibm.com>

	* doc/install.texi (Configuration): Document the
	--with-long-double-format={ibm,ieee} PowerPC configuration
	options.

From-SVN: r257473

gcc/ChangeLog

@@ -6,6 +6,10 @@
 
 2018-02-07  Michael Meissner  <meissner@linux.vnet.ibm.com>
 
+	* doc/install.texi (Configuration): Document the
+	--with-long-double-format={ibm,ieee} PowerPC configuration
+	options.
+
 	PR target/84154
 	* config/rs6000/rs6000.md (fix_trunc<SFDF:mode><QHI:mode>2):
 	Convert from define_expand to be define_insn_and_split.  Rework

gcc/doc/install.texi

@@ -1877,6 +1877,35 @@ When neither of these configure options are used, the default will be
 128-bit @code{long double} when built against GNU C Library 2.4 and later,
 64-bit @code{long double} otherwise.
 
+@item --with-long-double-format=ibm
+@itemx --with-long-double-format=ieee
+Specify whether @code{long double} uses the IBM extended double format
+or the IEEE 128-bit floating point format on PowerPC Linux systems.
+This configuration switch will only work on little endian PowerPC
+Linux systems and on big endian 64-bit systems where the default cpu
+is at least power7 (i.e. @option{--with-cpu=power7},
+@option{--with-cpu=power8}, or @option{--with-cpu=power9} is used).
+
+If you use the @option{--with-long-double-64} configuration option,
+the @option{--with-long-double-format=ibm} and
+@option{--with-long-double-format=ieee} options are ignored.
+
+The default @code{long double} format is to use IBM extended double.
+Until all of the libraries are converted to use IEEE 128-bit floating
+point, it is not recommended to use
+@option{--with-long-double-format=ieee}.
+
+On little endian PowerPC Linux systems, if you explicitly set the
+@code{long double} type, it will build multilibs to allow you to
+select either @code{long double} format, unless you disable multilibs
+with the @code{--disable-multilib} option.  At present,
+@code{long double} multilibs are not built on big endian PowerPC Linux
+systems.  If you are building multilibs, you will need to configure
+the compiler using the @option{--with-system-zlib} option.
+
+If you do not set the @code{long double} type explicitly, no multilibs
+will be generated.
+
 @item --enable-fdpic
 On SH Linux systems, generate ELF FDPIC code.